<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>对齐</title>
        <style>
            #bg{width:200px;height:40px;background-color:rgb(100,200,100);}
            #bg2{width:200px;height:40px;background-color:rgba(100,200,100,0.5)}
        </style>
    </head>
    <body>
        <div id="bg"></div>
        <br>
        <div id="bg2"></div>
    </body>
</html>